This invention relates to the partial oxidation of hydrocarbons, and more particularly to the production of hydrogen and carbon monoxide by the oxidation of hydrocarbons. Specifically, the invention relates to the high temperature adsorption of oxygen onto ceramic adsorbents and partial oxidation of hydrocarbons by contacting the hydrocarbons with the sorbed oxygen at elevated temperatures.
Syngas and its components, hydrogen and carbon monoxide, are conventionally produced by the high temperature partial oxidation of hydrocarbons with controlled amounts of air or oxygen. Although air is less expensive and more convenient to use in partial oxidation reactions, it is less attractive than oxygen for such reactions because the large quantities of nitrogen that are produced when air is used as the oxidant must be subsequently separated from the product gas prior to its use. The cost of gas separation and purification equipment required to purify the product gas adds considerably to the cost of syngas production using air.
Although oxygen is more desirable than air as an oxidant for partial oxidation reactions, its use is not without disadvantage, in that oxygen must be imported into the system, or it must be generated on site, for example, by means of a cryogenic air separation plant or an adsorption system. In either alternative, using oxygen as the oxidant likewise adds considerably to the cost of the process.
More economical methods of on site production of oxygen for applications such as hydrocarbon partial oxidation reactions are continuously sought. U.S. Pat. No. 5,714,091 discloses an oxygen-based hydrocarbon partial oxidation process in which the oxygen is produced on site by subjecting air to membrane separation using a membrane constructed of perovskite-based ceramic material. Oxygen, which is permeable to the membrane, passes through the membrane and is made to react with hydrocarbons on the downstream side of the membrane unit. The disadvantages of this method of oxygen production are the high cost of production of the membrane and the difficulty of producing membrane structures that are leak-proof.
The present invention provides a system and process for the partial oxidation of hydrocarbons with oxygen that is produced from air in the partial oxidation reactor using a relatively inexpensive ceramic-based adsorbent material and a simple reactor design. The method of this invention has the additional advantage that the heat produced by the partial oxidation reaction can be used to increase the overall efficiency of the process by maintaining the adsorbent at the desired adsorption temperature without an external heat source.
According to a broad embodiment, the invention comprises a process for producing hydrogen and carbon monoxide by the partial oxidation of at least one hydrocarbon comprising the steps:
(a) passing an oxygen-containing gas at a temperature in the range of about 300 to about 1400xc2x0 C. and at an absolute pressure in the range of about 0.5 to about 50 bara through at least one reaction zone containing an oxygen-selective mixed conductor, thereby preferentially adsorbing oxygen from said oxygen-containing gas; and
(b) passing the at least one hydrocarbon through the at least one reaction zone at a temperature in the range of about 300 to about 1400xc2x0 C., thereby producing a product gas comprising hydrogen, carbon monoxide or both hydrogen and carbon monoxide.
In a preferred embodiment of the invention, the oxygen-selective mixed conductor is selected from: (1) perovskite substances having the structural formula A1-xMxBO3-xcex4, where A is a rare earth ion, M is Sr, Ca, Ba or mixtures of these, B is Co, Mn, Cr, Fe or mixtures of these, x varies from 0 to 1 and xcex4 is the deviation from stoichiometric composition resulting from the substitution of Sr, Ca and Ba for rare earth ions; (2) ceramic substances selected from Bi2O3, ZrO2, CeO2, ThO2, HfO2 and mixtures of these, the ceramic substance being doped with CaO, rare earth metal oxides or mixtures of these; (3) brownmillerite oxide; and (4) mixtures of any of these,
In a more preferred embodiment of the invention, the oxygen-selective mixed conductor is a perovskite substance, and in a preferred aspect of this embodiment, x varies from about 0.1 to 1.
In another preferred embodiment, the oxygen-selective mixed conductor is a ceramic substance of group (2), above, and the ceramic substance is doped with a rare earth metal oxide selected from Y2O3, Nb2O3, Sm2O3, Gd2O3 and mixtures of these.
In another preferred embodiment, the oxygen-containing gas is air.
In another preferred embodiment, the process comprises repeatedly performing steps (a) and (b) in sequence in the above-mentioned at least one reaction zone.
In another preferred embodiment, the process further comprises, between steps (a) and (b), the additional step of removing nonadsorbed gas component from the adsorption vessel(s): (1) by purging the at least one reaction zone with gas that is compatible with the partial oxidation reaction product gas, (2) by depressurizing the at least one reaction zone or (3) by both purging the at least one reaction zone with gas that is compatible with the partial oxidation reaction product gas and depressurizing the at least one reaction zone. In a preferred aspect of this preferred embodiment, the gas that is used to purge the at least one reaction zone is oxygen, steam, carbon dioxide or mixtures of these.
In another preferred embodiment, the process further comprises, after step (b), removing residual product gas from the at least one adsorption zone: (1) by purging the at least one reaction zone with steam, carbon dioxide, nitrogen, argon, helium or mixtures of these, (2) by depressurizing the at least one reaction zone or (3) by both purging the at least one reaction zone with steam, carbon dioxide, nitrogen, argon, helium or mixtures of these and depressurizing the at least one reaction zone.
In another preferred embodiment, the at least one hydrocarbon has an aliphatic, cycloaliphatic or aromatic structure and it contains 1 to 12 carbon atoms.
In another preferred embodiment, the process is carried out at a temperature in the range of about 600 to about 1200xc2x0 C.
In another preferred embodiment, step (a) of the process is carried out at an absolute pressure in the range of about 0.5 to 20 bara.
In a more preferred embodiment, the at least one hydrocarbon contains 1 to 6 carbon atoms.
In other preferred embodiments, the oxygen-selective mixed conductor is a perovskite substance and A is La, Y or mixtures of these and/or M is Sr, Ca or mixtures of these and/or B is Co, Fe or mixtures of these. More preferably, A is La, Y or mixtures of these, and/or M is Sr, Ca or mixtures of these and/or B is Co, Fe or mixtures of these.
In another preferred embodiment of the invention the oxygen-selective mixed conductor is a perovskite substance and x is 0.2 to 1.
In a most preferred embodiment, the process is carried out at a temperature in the range of about 750 to about 1100xc2x0 C.
In a more preferred embodiment, the at least one hydrocarbon comprises 1 to 4 carbon atoms, and in one most preferred embodiment, it is methane. In another most preferred embodiment, the hydrocarbon feed gas is natural gas.
In another preferred embodiment, the at least one hydrocarbon comprises a petroleum derivative. In a more preferred embodiment, the petroleum derivative is naphtha, gasoline or mixtures thereof.
In another preferred embodiment, the at least one reaction zone contains particulate material having a thermal conductivity greater than that of the oxygen-selective mixed conductor. In one preferred aspect of this preferred embodiment, the high thermal conductivity particulate material is mixed with the oxygen-selective mixed conductor, and in another preferred aspect, the high thermal conductivity particulate is placed, upstream, downstream or both upstream and downstream of the oxygen-selective mixed conductor.
In another preferred embodiment, the at least one reaction zone additionally contains one or more catalysts for partial oxidation reactions. In a more preferred embodiment, the additional catalyst is deposited on the oxygen-selective mixed conductor.
In another preferred embodiment, the process of the invention further comprises, during step (b), passing a moderating agent selected from steam, carbon dioxide and mixtures of these through the at least one reaction zone. In a preferred aspect of this embodiment, the moderating agent is steam.